The rapidly increasing use of fossil fuels worldwide depletes the finite supply and raises major concern over the associated greenhouse gases emissions and air pollutants. As reported by the United States Environmental Protection Agency, the emission of particulate matter from burning B20 (20% by volume of biodiesel and 80% by volume of petroleum diesel mixture) would decrease by approximately 10% in comparison with emission from burning regular fuel, such as diesel. Also, the emission of carbon monoxide and hydrocarbons would decrease 21.1% and 11%, respectively. As a result, the demand for renewable energy in the form of biodiesel as an alternative has increased dramatically. Approximately 2.1 billion gallons of biodiesel have been introduced into the American fuel market in 2016. Purified biodiesel, which meets the biodiesel standard (ASTM D 6751) can be directly used in the diesel engine. Modification of diesel engine is not essential when running with biodiesel fuel. Furthermore, most major engine companies affirm in their Original Equipment Manufacturers (OEMs) that using blends up to B20 will not void the engine warranties.
The major feedstock for the biodiesel industry in the United States includes soybean oil, canola oil, white/yellow grease, and tallow and many more. Moreover, 54.32% of total biodiesel feedstock consumed in December 2013 came from soybean oil. With the growing demand for soybeans in both food and fuels, the price of soybeans has increased as well. This price pressure is relevant because the feedstock cost for producing biodiesel is approximately 70%-95% of the total cost leading to a high sales price for biodiesel. The price of biodiesel hit a historic high of $4.81 per gallon in 2008. Because of the economic downturn, Renewable Fuel Standard (RFS) 2 uncertainty, and the lapse of the biodiesel tax credit, the price of biodiesel reached a new record. In order to make the biodiesel price more competitive with diesel price, studies to assess the feasibility of using inexpensive waste materials as feedstock for biodiesel production need to be undertaken.
Coffee is the second largest traded commodity worldwide. The world's coffee production in 2016/2017 is estimated to be 9.54 million tones according to USDA's report, and coffee consumption in the United States is approximately 1.7 million tons. Up to 0.91 g of spent coffee grounds (SCGs) can be generated per gram of coffee. According to a recent study, 8%-20% by weight of oil is found within SCGs. Also, SCGs have a minimum cost of acquisition, and hence, if used as an alternative feedstock for biodiesel production, would reduce the high price of feedstock in the biodiesel industry. Since the SCGs oil proportion is similar to the soybean oil percentage (about 20% by weight), SCGs have sufficient oil content to be used as a feedstock to produce biodiesel.
During biodiesel production, purification is necessary to purify the crude biodiesel. The most common materials that are used during biodiesel purification are Purolite PD206 ion exchange resin and magnesol adsorbents. However, these two types of materials are expensive. In order to minimize the cost of biodiesel production thereby increasing its demand, lower cost purification materials are required.
Reutilizing the SCGs have also been considered as biodiesel feedstock, as well as solvent recovery, where coffee oil was first extracted from SCGs by a solvent extraction process. Solvents such as hexane or a hexane/isopropanol mixture are commonly used in the process. After oil extraction, a two-step process, acid esterification followed by alkaline transesterification, is performed to convert the oil into biodiesel. One such process is shown in FIG. 1 where SCGs are gathered and dried to remove water. Solvents, such as hexane and isopropyl alcohol (IPA) is mixed with the dried SCGs and thereafter recovered to produce coffee oil which is separated from the residual grounds (solids). The coffee oil is them mixed with additional solvents such as methanol (MeOH) and sulfuric acid (H2SO4) and subjected to an acid-base esterification reaction. The methanol is then recovered and the remaining material is mixed with potassium hydroxide solvent (KOH) and subjected to a transesterification reaction to produce biofuel and glycerin. Unfortunately, such processes require two solvent recovery processes which are relatively expensive due to the need to use technology such as vacuum distillation. As shown, first such process is needed to remove the oil extraction solvents, such as hexane or a mixture of hexane and isopropyl alcohol. While the mixture solvent is used to obtain higher coffee oil extraction efficiency, it makes recycling more difficult due to different boiling points/vapor pressures. The second process is needed to complete the process of producing biofuel. Unfortunately, the complexity in this three-step process (solvent extraction, esterification and transesterification) to make biodiesel from SCGs is costly to adopt and reduces the ability of biodiesel from spent coffee to be economically competitive. Therefore, the solvent extraction process is viewed as a “tough sell” to the biodiesel industry due to its high cost and added safety requirements in chemical handling.
Another process for producing biodiesel from spent coffee grounds is shown and described in U.S. Pat. No. 8,591,605 that uses a solvent extraction method to obtain coffee oil. Solvent is removed, such as by boiling off or vacuum distillation, and a transesterification reaction is used to create biodiesel. Unfortunately, this system has limited use due to disadvantages of the solvent extraction process similar to other published journal papers.
One approach is described in U.S. Pat. No. 7,612,221 whereby fatty acid alkyl esters were produced by transesterifying a feedstock containing lipid-linked fatty acids with an alcohol and an alkaline catalyst to form the fatty acid alkyl esters. However, this patent is focused on intact plant seeds and fruits, and include soy, coconut, corn, cotton, flax, palm, rapeseed/canola, safflower, sunflower and the like. In addition, it also uses an alkaline catalyst to form the fatty acid alkyl esters. Other approaches used a combination of solvents, such as methanol and chloroform to create a transesterification reaction with spent coffee grounds. However, the use of methanol and chloroform co-solvents makes solvent recovery more expensive and the use of recovery methods, such as vacuum evaporation often results in changes to the final composition. Further, the use of such co-solvents makes the process more difficult for large volume production.
SGCs have also been converted into activated carbon which have been used to capture water pollutants, including H2S, organic pollutants such as p-nitrotoluene and n-nitrophenol and methylene blue (to represent dyes). SCGs have a low ash content of 3.5 wt. %, which is much lower than other agricultural waste which favors micro-porosity. Biochar is becoming increasingly recognized as a significant resource to inexpensively increase agricultural production efficiency.
Bio-char does this by: (1) increasing soil moisture retention; (2) moderating soil acidity; (3) increasing soil nutrient retention; (4) reduced leaching of beneficial nitrogen into ground water; (5) increasing soil microbial activity; and (6) decreasing plant pathogens. Because biochar does not undergo the “activation” step which is required for the production of high surface area carbons, they typically have residual carboxylic acid functionalities remaining in their structure which results in a significant amount of ion exchange capacity. This cation exchange capacity is primarily responsible for the improvements in increased soil fertility and crop yields.
There are several environmental benefits of biochar including the retention of phosphorous to reduce contamination of surface waters due to nutrient runoff, and biochar is considered carbon negative and has potential as a CO2 sequester material Biochar also has the advantage of being produced at a significantly lower price than other soil amendments with similar benefits. Further, biochar has the potential to be produced in bulk quantities relatively cheaply because of the wide variety of high volume biomass material that can be used as feedstock. The low ash (less than 4%) and low contaminant level of SCGs are expected to provide a higher yield of final product when compared to other feedstocks. Biochar produced from products such as SCG available in densely populated cities can be used locally as an amendment for increasing common urban farming, vegetative roofs, and indoor growing facilities and thereby reduce the transportation cost of the raw materials and of the final product to users. Mild pyrolysis (e.g., temperatures less than 600° C.) is generally employed to produce biochar and have been shown to be the most effective production method. Pyrolysis is the heating of degradable material in the absence of oxygen, and results in a high carbon containing solid product. Pyrolysis has been used extensively for the production of activated carbon, and for producing biochar from plant materials. However, until now, there has not been an optimal process developed to efficiently produce high quality biochar from SCGs.
In view of the foregoing, it would be desirable to have a process of producing biofuel from spent coffee grounds (SCGs) that is significantly less costly and time consuming, and whereby residual SCG solids are developed into carbon products, such as biochar or activated carbon, which further increases the financial feasibility of utilizing SCGs.